Individuals continuously exposed to malaria infections in endemic areas develop immune responses that have been shown to reduce or block parasite transmission from patients to the mosquito vector in what is called transmission-blocking (TB) immunity. In this context, antibodies (Abs) targeting specific Plasmodium antigens expressed on gametocyte, zygote, and ookinete stages can induce this blockage, providing the bases for the development of TB vaccines. One of these antigens, P48/45 is a conserved protein expressed in all Plasmodium species, required for parasite fertilization, and currently being developed as a vaccine candidate for P. falciparum. The P. vivax orthologue, cloned and expressed in E. coli, as a ~60kDa recombinant product has been shown to be highly antigenic and immunogenic. Individuals exposed to P. vivax produce Abs to Pvs48/45 which increases in an age dependent manner, and those individuals with higher a-Pvs48/45 Ab titers also display high ex vivo TB activity. Pilot studies have shown that the rPvs48/45 protein is highly immunogenic both in mice and monkeys and Abs elicited have the capacity to ex vivo block parasite transmission to mosquitoes. Together, all these data make Pvs48/45 a very promising TB vaccine candidate. The goal of this proposal is to determine the vaccine potential of rPvs48/45 for human use. We will further characterize the antigenicity of rPvs48/45 using sera and cells of individuals from endemic areas, and rPvs48/45 domains and synthetic peptides to map the relevant immune and functional epitopes. Additionally, we will confirm and further characterize the immunogenicity of full length Pvs48/45 and selected fragments in non-human primates. Furthermore, the 3D structure data of the protein(s) will be obtained by physico-chemical analyses [circular dichroism (CD) and nuclear magnetic resonance (NMR) spectroscopies and mass spectrometry (MS) analysis], and bioinformatics and molecular modeling. Natural Abs induced in individuals exposed to P. vivax and P. falciparum in endemic areas of Colombia, Brazil and Burkina Faso will be assessed by ELISA and T-cell responses by flow cytometry. Sera and affinity purified IgG will be tested for TB capacity in MFAs. Furthermore, immunogenicity studies in mice and monkeys will address the potential cross prime/boost effect of parasite infection on Ab response elicited by immunization and vice versa. Potential cross-reactivity of elicited Abs against P. falciparum will be also tested. All facilities and techniques required are available and routinely conducted by the participant groups. At the end of the study we expect to have identified the optimal conditions for Pvs48/45 to induce effective TB immunity in humans and animals and the best vaccine formulations for further clinical development.